Project Summary ? Project #2 Similar to most organs, the various functions of the skeleton are influenced by extracellular cues, hormones and neurotransmitters. One type of neuronal regulation of bone mass is the one initiated by the neurotransmitter serotonin that favors bone mass accrual by inhibiting the activity of the sympathetic nervous system. This observation raises questions about the transcriptional mechanisms regulating brain serotonin accumulation and catecholamine synthesis. In addressing this question we have found that the histone deacetylase Sirt1 is a transcriptional modulator of the neuronal control of bone mass. A generalized but modest increase in Sirt1 expression (TgSirt1) in transgenic mice compromises bone mass by suppressing bone formation and by promoting bone resorption. The opposite effects of Sirt1 on the two compartments of bone remodeling correlate with an increase in the activity of the sympathetic nervous system (SNS). Three additional observations implicated the SNS as a mediator of the skeletal actions of Sirt1. First, Ucp1 expression in brown fat and catecholamine levels are increased in TgSirt1 mice. Second, pharmacological inhibition of the SNS activity in TgSirt1 mice fully restores osteoblast and osteoclast numbers and rescues the low bone mass phenotype of these animals. Third, and more directly, adenovirus-mediated deletion of Sirt1 in the brain decreases SNS activity and increases bone mass by increasing bone formation and suppressing bone resorption. Hence, our data suggest that neuronal Sirt1 controls bone mass by increasing SNS signaling. Sirt1 may do so by increasing expression of monoamine oxidase A (MAO-A), a Sirt1 target that reduces serotonin levels in the brain and/or by suppressing serotonin synthesis in the brainstem or by modulating sympathetic tone in the locus coeruleus. This application intends to identify Sirt1 site(s) of action in the brain, osteoblasts. osteocytes and osteoclasts in the aging skeleton, to demonstrate genetically that the SNS mediates the skeletal effects of Sirt1, to determine whether brainstem, locus coeruleus or MAO-A-expressing neurons is the specific brain site(s) through which Sirt1 regulates skeletal homeostasis and to evaluate the respective contribution of MAO-A and Tph2 in the Sirt1- dependent regulation of bone mass.